1. Field of the Invention
The invention relates to water-dispersible hybrid polymer comprising an ethylenically unsaturated polyurethane (core) onto which an addition polymer chain (shell) has been provided by polymerization, to an aqueous dispersion incorporating such a polymer, and to the use of such a dispersion in coating compositions.
2. Description of the Prior Art
A water-dispersible hybrid polymer of the aforementioned type is known from WO-A-91/15 528. In WO-A-91/15 528, the ethylenically unsaturated groups in the polyurethane originate from a polyester polyol and/or a polyether polyol and/or a compound which in addition to an ethylenically unsaturated group shows a group reactive toward NCO groups. As examples of suitable ethylenically unsaturated compounds may be mentioned trimethylol propane monoallyl ether, glycerol monoallyl ether, pentaerythritol diallyl ether, and alkenedicarboxylic acids, such as maleic acid and unsaturated dimerized fatty acids. The compositions disclosed in said document are recommended exclusively for use in coating compositions of the so-called base coat type. They were found to be less suitable for use in coating compositions of the so-called clear coat type, particularly when stringent demands are imposed with regard to the acid resistance of topcoats applied using these compositions.